It's really depends on your relationship with your manager. 99% of the time, it's a bad idea to say you're interviewing. If your manager is someone you trust, and they genuinely have your interests in mind, you can talk to them and they can use their influence to try to solve whatever problem is making you think about leaving.
If you're not confident that's the case (and unfortunately it's rare), it's too risky to make it known you're interviewing. It is often better to discuss whatever is making you unhappy, but without saying that you are considering leaving because of it.
If you're not confident that's the case (and unfortunately it's rare), it's too risky to make it known you're interviewing. It is often better to discuss whatever is making you unhappy, but without saying that you are considering leaving because of it.